Crucible with differentially expanding release mechanism

ABSTRACT

A crucible is held in a closed position when the crucible is at a certain temperature. A temperature sensitive member expands differently in response to heat than other portions of the crucible. When the temperature of the temperature sensitive member is increased, the temperature sensitive member expands an amount different than do other portions of the crucible and thereby causes the crucible to open.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to the growth of doped semiconductor crystalsand, in particular, the doping of a melt as part of the process ofgrowing the doped semiconductor crystals.

2. Description of Related Art

Semiconductor wafers can be cut from semiconductor crystals grown in aCzochralski-type crystal-growing furnace. The semiconductor waferstypically include a controlled concentration of a dopant to producedesired electrical characteristics. The dopants are typically added to amelt such as, for example, molten silicon, in the crystal-growingfurnace. The silicon, for example, and the dopant mix together in aliquid state to produce a molten mixture having the desired dopantconcentration. A single-crystal ingot is pulled from the molten mix.

Some dopants, such as antimony, have a vapor pressure sufficiently highto cause the concentration of dopant in the melt to change significantlyas the silicon, for example, and dopant are melting together in thefurnace. The change in dopant concentration cannot be accuratelypredicted, so the ability to accurately produce crystals having desiredelectrical properties is limited. To overcome this problem, severaltechniques have been developed for adding high vapor pressure dopants tothe semiconductor after it is melted. Such techniques are difficult toperform, however, because the molten semiconductor (the "melt") must bemaintained in an inert atmosphere in the crystal-growing furnace.

A Czochralski-type crystal-growing furnace typically includes twoseparately sealed vacuum-tight chambers. The upper chamber, or pullchamber, has space for enclosing the ingot as it is grown and includes aseed cable or shaft for lowering and raising a seed crystal. The lowerchamber, or furnace tank, includes a crucible containing the melt.

Several methods for adding the dopant to the melt are known. Forexample, the melt can be doped by attaching a dopant to a seed crystaland then dipping the seed and dopant into the melt. However, dipping theseed into the melt, without appropriate preheating, causes thermalstress within the seed, which can result in cracks or structuraldefects. Preheating of the seed may cause premature dopant drop,resulting in a melt splash. Various methods for attaching the dopant tothe seed so as to prevent premature dopant drop are known. One suchmethod includes boring a transverse hole into the seed crystal, meltingthe dopant and then solidifying the dopant around the seed and in thehole. Alternatively, a ring of dopant can be placed over the seed andkept from dropping off by laser-welding a block of silicon to the bottomof the seed.

U.S. Pat. No. 5,406,905 shows a method of doping the melt by attachingthe dopant to the seed crystal and then lowering the dopant-seedassembly to just above the melt and holding it there while heat from themelt heats the seed and the dopant, allowing the dopant to slip off ofthe seed.

It is also known to form the dopant into a thin electricalcurrent-carrying wire attached to the seed crystal. The seed contactsthe molten semiconductor, causing the dopant wire to melt and therebyinterrupt the current flow. The interrupted current flow automaticallycauses a computer to begin the crystal-pulling procedure.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a crucible having a portion thatopens when subjected to an increase in temperature. At a giventemperature, the opening portion of the crucible is held in a closedposition. The crucible has a temperature sensitive portion that expandsdifferently in response to heat than at least some other portion of thecrucible. The different expansion is due to, for example, a temperaturesensitive member being made from a material that expands differently inresponse to heat than the material from which other portions of thecrucible are made. The differential expansion can also be caused by thestructure of the temperature sensitive member and the structure of theother portions of the crucible. When the temperature of the temperaturesensitive member is increased, the differential expansion of thetemperature sensitive member and the other portions of the cruciblecauses the opening portion of the crucible to open.

As applied to a crucible for doping a melt as part of the process ofgrowing doped semiconductor crystals, the temperature increase can becaused by heat radiating from the melt.

The present invention can avoid problems associated with known methodsand devices for delivering dopant to the melt in a crystal-growingfurnace by providing a simple device for containing the dopant andreleasing the dopant close to the surface of the melt. In particular, bydropping the dopant at a releasing point that is very close to thesurface of the melt, dopant particles are prevented from being blownaway from the melt by eddy currents in the gasses above the melt. Ifdopant particles are blown away from the melt, the desired concentrationof dopant in the crystal is not achieved and the resistivity of thecrystal is adversely affected. A releasing point close to the surface ofthe melt also helps prevent melt splash. Also, the dopant can becontained within an enclosed crucible and need not be exposed to theatmosphere inside the furnace until the dopant may be dropped into themelt and, therefore, evaporation of the dopant may be reduced. Inaddition, because the seed crystal need not be involved in the dopantdelivery process, it need not be subjected to rapid temperature changesthat could cause cracking and structural defects due to thermal stress.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of thepresent invention will be described in or apparent from the followingdescription of embodiments with reference to the accompanying drawings,wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a front elevational view of a first embodiment of theinvention;

FIG. 2 is a left side elevational view of the embodiment shown in FIG.1;

FIG. 3 is a partial front elevational view of the embodiment shown inFIGS. 1 and 2 showing the bottom of the device in an open position;

FIG. 4 is a front elevational view of a second embodiment of theinvention;

FIG. 5 is a left side elevational view of the embodiment shown in FIG.4;

FIG. 6 is a partial sectional view taken along section line VI--VI inFIG. 4 showing the bottom of the device in an open position;

FIG. 7 is a front elevational view of a third embodiment of theinvention;

FIG. 8 is a left side elevational view of the embodiment shown in FIG.7;

FIG. 9 is a partial sectional view taken along section line IX--IX inFIG. 7 showing the bottom of the device in an open position;

FIG. 10 is a front view of a fourth embodiment of the invention;

FIG. 11 is a front view of a fifth embodiment of the invention;

FIG. 12A is a front view of a sixth embodiment of the invention in aclosed position;

FIG. 12B is a front view of the sixth embodiment of the invention in anopen position;

FIG. 13A is a front view of a seventh embodiment of the invention in aclosed position;

FIG. 13B is a front view of the seventh embodiment of the invention inan open position;

FIG. 14 shows the first embodiment before being lowered to the surfaceof a melt;

FIG. 15 shows the first embodiment after being lowered to near thesurface of the melt and with the bottom of the device in an openposition; and

FIG. 16 shows the second embodiment after being lowered to near thesurface of the melt and with the bottom of the device in an openposition.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In embodiments of the present invention, a crucible made of a materialsuch as, for example, quartz has a bottom that opens by way of, forexample, a hinge. The bottom of the crucible is held in a closedposition by, for example, a latch. A temperature sensitive member orportion such as, for example, a pin is located, for example, between thelatch and a side of the crucible. The temperature sensitive member ismade of a material that expands differently in response to heat than thematerial from which other portions of the crucible are made.

Dopant to be added to the melt is placed in the crucible and thecrucible is lowered toward the surface of the melt. As the crucibleapproaches the surface of the melt, the temperature of the crucibleincreases due to the heat radiating from the surface of the melt.Because the temperature sensitive member expands differently than otherportions of the crucible, the temperature sensitive member moves someportion of the crucible and causes the latch to release the bottom ofthe crucible. Without the latch holding the bottom of the crucible, thebottom of the crucible is free to fall open and deliver the dopant tothe melt.

In other embodiments, the bottom of the crucible is made of multiplepieces held by one or more latches. Such embodiments operate in the samemanner as previously discussed, except that, because a multi-piecebottom is used, the crucible can be lowered closer to the melt beforethe bottom is opened without the bottom touching the melt in the openposition.

In other embodiments, the bottom of the crucible is held in the closedposition not by a latch, but by friction between the bottom of thecrucible and a main body of the crucible. In such embodiments, thebottom of the crucible is made from a material that expands differentlyin response to temperature changes than the material from which the mainbody of the crucible is made. When the crucible is at a temperaturelower than the temperature directly above the melt, the bottom and themain body of the crucible are forced together such that the bottom isheld in a closed position. When the crucible is heated by heat radiatingfrom the melt, for example, the bottom and the main body expanddifferent amounts because they are made from materials that expanddifferently in response to temperature changes. This differentialexpansion reduces the forces that hold the bottom closed so that thebottom opens and delivers the dopant to the melt.

According to the invention, a particular structural member of thecrucible can be designed so that it expands by a greater amount and/orat a faster rate than some other structural member so as to cause thecrucible to open when heated. Conversely, a structural member can bedesigned so as to expand by a lesser amount or at a slower rate thananother portion of the crucible so as to cause the crucible to open whenheated.

The invention can be practiced a number of different ways. For example,the material that is used to make a particular structural member can bea different material than that which is used to make another structuralmember, those two different materials expanding by different amountsand/or at different rates. Alternatively, or in combination with theselection of material aspect, a structural member can be designed so asto have a physical structure that will expand by a different amount orat a different rate than some other structural member. For example, afirst structural member having a lesser mass (for example because it isthinner) than a second structural member will heat-up at a faster ratethan the second structural member. This will cause the first structuralmember to expand at a faster rate than the second structural member.This technique can be used to design crucibles according to theinvention from a single type of material.

In a first embodiment of the present invention, as shown in FIGS. 1-3, acrucible 10 has a main body 12, a cover 14, and a bottom (door) 16 thatis attached to the main body 12 by a hinge 18. The crucible 10preferably is made of a material that will not introduce undesirablecontaminants into the furnace, such as, for example, clear naturalquartz.

The bottom 16 is held in a closed position by a latch 20, which pivotsabout a latch hinge 22. A temperature sensitive member such as, forexample, a release pin 24 is located between the latch 20 and the mainbody 12. The release pin 24 is made of a material (for example adifferentially expanding quartz or stainless steel) that expandsdifferently in response to heat than the material from which the mainbody 12 and latch 20 are made. The release pin 24 may be held in placeby a pin housing 26, or the like. The cover 14 can optionally be removedfrom the main body 12 and is attached to the main body 12 by a threadedconnection, a bayonet connection, or the like. The crucible has a shaft28, or similar connector, that connects the crucible to a mechanism (notshown) that raises and lowers the crucible 10 within the furnace. Theshaft 28 can be configured so as also to fit into the mechanism thatlowers the seed crystal into the melt, thereby avoiding the need for aseparate raising and lowering mechanism.

Dopant to be added to the melt 42 can be placed in the crucible 10 byremoving the cover 14 and placing the dopant onto the closed bottom 16of the crucible 10. As shown in FIGS. 14 and 15, the crucible 10 islowered toward the surface of the melt 42, which is contained within themelt crucible 40. As the crucible 10 approaches the surface of the melt42, the temperature of the crucible 10 increases due to the heatradiating from the surface of the melt 42. Because the release pin 24expands differently (at a greater rate) than the crucible 10 and latch20, the release pill 24 moves the latch 20 (to the left in FIG. 3) andcauses the latch 20 to release the bottom 16 of the crucible 10. Withoutthe latch 20 holding the bottom 16 of the crucible 10, the bottom 16 ofthe crucible 10 is free to fall open (as shown in FIGS. 3 and 15) anddeliver the dopant to the melt 42.

In a second embodiment, the bottom of the crucible 10 is made ofmultiple pieces 16' and 16' held in the closed position by one or morelatches 20. In the example shown in FIGS. 4-6 and 16, the bottom is madeof two pieces 16' that are held in the closed position by one latch 20.The second embodiment operates in the same manner as the firstembodiment except that, because a multi-piece bottom (door) is used, thecrucible 10 can be lowered closer to the melt 42 before the bottom isopened without the bottom touching the melt 42 in the open position.This is because when a multi-piece bottom is in the open position (asshown in FIGS. 6 and 16), the distance d₂ between each end portion 17'of the bottom 16' and the lower end 13 of the main body 12 is less thanthe corresponding distance d₁ in the first embodiment. For example, inthe case of the bottom having two pieces 16' (as shown in FIG. 6), d₂ isapproximately 1/2d₁.

In a third embodiment (shown in FIGS. 7-9), the bottom pieces 16" of thecrucible 10 are held in the closed position not by a latch, but byfriction between the bottom pieces 16" of the crucible 10 and the mainbody 12 of the crucible 10. In this embodiment, the bottom pieces 16" ofthe crucible 10 are made from a material that expands differently inresponse to temperature changes than the material from which the mainbody 12 of the crucible 10 is made. When the crucible 10 is at atemperature lower than the temperature directly above the melt 42, thebottom pieces 16" and the main body 12 of the crucible 10 are forcedtogether such that the bottom pieces 16" are held in a closed position.In the closed position, a first engaging piece 30 on one end portion 17"of one bottom piece 16" engages a second engaging piece 32 on the otherend portion 17" of the other bottom piece 16". When the crucible 10 isheated by heat radiating from the melt 42, for example, the bottompieces 16" and the main body 12 expand different amounts (because theyare made from materials that expand differently in response totemperature changes). This differential expansion reduces the forcesthat hold the bottom pieces 16" in the closed position so that thebottom pieces 16" (i.e., the door) open and deliver the dopant to themelt 42.

Although FIGS. 8 and 9 show the first engaging piece 30 on one endportion 17" of bottom piece 16" and the second engaging piece 32 on theother end portion 17" of bottom piece 16", it is noted that end portions17" can be configured in any shape that provides both a sufficient sealto prevent dopant from falling from the crucible while the bottom pieces16" are in the closed position and a sufficient bearing surface for thefriction force that keeps the bottom pieces 16" in the closed position.

Although this third embodiment has been explained with reference to anexample where the forces that hold the bottom pieces 16" in the closedposition are between the bottom pieces 16" and the main body 12, theforces that hold the bottom pieces 16" in the closed position can,instead, be between differently expanding pieces of a multi-piecebottom.

In addition, although this third embodiment has been explained withreference to a bottom having two pieces 16", the bottom can have onlyone piece or more than two pieces.

In the third embodiment, the bottom pieces 16" may be made from amaterial that expands by a lesser amount or at a slower rate than themain body 12 so that the tightness of the fit between the bottom pieces16" is reduced during heating to allow the bottom to open.Alternatively, the bottom pieces 16" may be made from a material thatexpands by a greater amount or at a faster rate than the main body 12 sothat the bottom pieces pop open during heating.

In a fourth embodiment, as shown in FIG. 10, the latch 20' is attachedto a latch hinge support 22A by the latch hinge 22. A release pin 24' isattached at one end to the main body 12 by a pin housing 26' and isattached at the other end to the latch 20'. Release pin 24' is attachedto the latch 20' such that expansion of hinge support 22A relative torelease pin 24' causes latch 20' to pivot about latch hinge 22. Releasepin 24' and latch hinge support 22A are made such that they expand bydifferent amounts when subjected to a temperature change. In particular,the release pin 24' expands by a lesser amount and/or at a slower ratethan the latch hinge support 22A when heated. The differential expansioncan be caused, for example, by the release pin 24' and the latch hingesupport 22A being made from different materials or having differentstructures, as well as by latch hinge support 22A shielding release pin24' from the heat of the melt.

In a fifth embodiment, as shown in FIG. 11, the movable latch shown inseveral other embodiments is replaced by a fixed latch 20". In thisembodiment, a bottom extension 16A is fixed to the bottom 16 of thecrucible as shown, for example, in FIG. 11. Below a particulartemperature, a free end of the latch 20" and a free end of the bottomextension 16A overlap such that the bottom 16 is prevented from opening.When the temperature of the crucible is increased, the latch 20" expandsa different (greater) amount than the bottom extension 16A such that thefree end of latch 20" and the free end of bottom extension 16A no longeroverlap, thereby allowing bottom 16 to open.

In a sixth embodiment, the main body of the crucible has two main bodyportions 12' and 12' as shown in FIGS. 12A and 12B. The main bodyportions 12' are connected to a common hinge 18' which is, in turn,supported by a shaft 28'. The hinge 18' is a pivot point about which oneor both of the main body portions 12' pivot. In a closed position, asshown in FIG. 12A, the main body portions 12' can contain a materialsuch as, for example, the dopant. Each end of a release pin 24" isattached to a different one of the main body portions 12' by way of apin housing 26". As the temperature of release pin 24" is increased,release pin 24" expands and causes the main body portions 12' to rotateabout hinge 18' and away from each other, allowing the material insidethe crucible to fall from the crucible. The differential thermalexpansion of the release pin 24" and the main body portions 12' can becaused by, for example, the release pin 24" being made from a differentmaterial than the main body portions 12' or the structure of release pin24" relative to the structure of the main body portions 12'.

In a seventh embodiment, as shown in FIGS. 13A and 13B, the crucible hasa similar structure to the sixth embodiment except that the release pin24'" is located above and connected to the main body portions 12' by wayof pin housings 26'" connected to a pair of connecting members 36 thatare themselves connected at a pivot 34.

Additionally, portions that open upon heating need not be located on thebottom of the crucible as shown in the illustrated embodiments. Forexample, a door could be located on a lower side portion of the crucibleand the bottom surface of the crucible could be inclined so that thedopant slides out of the crucible when the side door opens.Additionally, while gravity is relied upon for opening the door andreleasing the dopant in several of the depicted embodiments, it ispossible to include a biasing mechanism or material, for example, topropel the door open and expel the dopant. However, embodiments arepreferred that have a simple structure, which does not introducecontaminants into the furnace.

While the present invention has been described with reference toembodiments thereof, it is to be understood that the invention is notlimited to the disclosed embodiments or constructions. To the contrary,the invention is intended to cover various modifications and equivalentarrangements. In addition, while the various elements of the disclosedinvention are shown in various combinations and configurations, whichare exemplary, other combinations and configurations, including more,less or only a single element, are also within the spirit and scope ofthe invention.

What is claimed is:
 1. A crucible for delivering a dopant to asemiconductor melt in a semiconductor crystal-growing furnace, thecrucible comprising:a chamber having an opening portion that reversiblycloses the chamber; and a temperature sensitive release mechanism havinga differentially expanding portion, the differentially expanding portionexpanding differently from another portion of the release mechanism whenthe release mechanism is heated from a first temperature to a secondtemperature greater than the first temperature, to move at least aportion of the release mechanism, thereby causing the release mechanismto open the chamber.
 2. The crucible of claim 1, wherein the chamber ofthe crucible is made of quartz.
 3. The crucible of claim 2, wherein thedifferentially expanding portion is made of quartz.
 4. The crucible ofclaim 2, wherein the differentially expanding portion is made ofstainless steel.
 5. The crucible of claim 2, further comprising a shaftreleasably attached to the chamber, the shaft being attachable to achuck that holds a seed crystal for growing a semiconductor crystal inthe semiconductor crystal-growing furnace.
 6. The crucible of claim 1,wherein the release mechanism includes a latch, the latch holding theopening portion in a closed position when the latch is in a firstposition and releasing the opening portion when the latch is in a secondposition, wherein the differentially expanding portion moves the latchfrom the first position to the second position when the differentiallyexpanding portion is heated from the first temperature to the secondtemperature.
 7. The crucible of claim 6, wherein the differentiallyexpanding portion is a pin located between the chamber and the latch. 8.The crucible of claim 6, wherein the differentially expanding portion ismade from a material having a first coefficient of thermal expansion andthe latch is made from a material having a second coefficient of thermalexpansion different than the first coefficient of thermal expansion. 9.The crucible of claim 8, wherein the first coefficient of thermalexpansion is larger than the second coefficient of thermal expansion.10. The crucible of claim 1, wherein the differentially expandingportion is a door of the chamber.
 11. The crucible of claim 10, whereinthe door includes at least two pieces.
 12. The crucible of claim 1,wherein the chamber has two wall parts and the release mechanismseparates the two wall parts when the differentially expanding portionis heated to the second temperature.